The present invention relates to apparatus and method for forming a bead on the border of a swimming pool liner or similar article of plastic sheet material.
Many swimming pools located partially or totally above ground comprise a plastic liner which is suspended from a frame circumscribing the pool. To facilitate mounting the swimming pool liner to the frame, the liner is provided with a bead along its border which engages a corresponding slot in the frame. After the bead has been so inserted it is locked in position by a spacer which is forced into the slot over the border of the sheet material including the bead.
The bead is ordinarily fabricated into the border of the plastic sheet material forming the swimming pool liner by heat sealing an additional strip of bead forming material to the border. However, when two pieces of plastic material are heat sealed together, weak spots are formed on the two sides of the heat sealed seam. This is a particular problem with the inside edge of the seam because the liner is suspended from the bead and large stresses are exerted on the sheet material where it is connected to the bead. These large stresses are focused at the weak spot formed on the inside edge of the seam. As a result, the bead material can become separated from the liner at this point and fail prematurely. In the past this problem has appeared endemic to heat sealing processes and the resultant weak point in the swimming pool construction has been merely accepted as a difficulty inherent in such construction.